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Overview By name, an embedded system is a special-purpose computing device designed to perform dedicated functions. Some of the embedded systems with real-time constraints are called real-time embedded systems. An embedded system consists of its hardware and software. The hardware includes microprocessor or microcontroller with additional attached external memory, I/O, and other components such as sensors, keypad, LEDs, LCDs, and any kind of actuators. The embedded software is the driving force of an embedded system. Most real-time embedded system software have specific application programs supported by Real Time Operating System(RTOS). The embedded software is usually called firmware because this type software is loaded to ROM, EPROM, or Flash memory and once it is loaded and it will never be changed unless it needs to be reloaded for replacement. The embedded systems are only used to repeatedly carry particular designated function.

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Categories of Embedded Systems 1.Stand-alone embedded systems A stand-alone embedded system works alone itself. It is a self- contained device. It takes either digital or analog inputs from its input ports, calibrates, coverts, and processes the data, and outputs the result data to its attached output device either displays data or controls and drives the attached devices. The entertainment devices of video game console and MP3, digital camera, and microwave are such typical systems fall into this category.

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2. Real-time embedded systems A system is said to be real-time if the response time is critical. In other word, some particular work must be done in a specified time period. In addition to functional correctness, the time constraint must be satisfied. There are two type real-time embedded systems: hard real-time and soft real-time embedded systems. i.Hard real-time systems. For a hard or immediate real-time system, the completion of an operation after its deadline may lead to a critical failure and result in live loss and property damage. The response time deadline for such system is very critical (in milliseconds or even shorter). These systems usually interact directly with physical hardware instead of human being. You can even find hard real-time embedded system in your daily life.

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For example: a car airbag control system is a hard real-time system because a delayed reaction may cause driver’s life and the delay response in heart pacemakers will also lead a severe result. It is imperative for such system to react an event within a strict deadline and missing a deadline will constitutes failure of the system. Hardware and software of hard real-time systems must allow a worst case execution (WCET) analysis that guarantees the execution be completed within a strict deadline. ii Soft real-time system In some other embedded system, there is response deadline but the lateness can be tolerated in some degrees. The violation of time constraints will result in degraded quality, but the system can continue to operate. Microwave and washing machine fall into this sub-category. Although there is a response time deadline for any operation the allowed latency delay can be seconds rather than ms. In soft real-time systems, the design focus is to offer a guaranteed bandwidth to each real-time task and allocate a percentage of the resources to each task.

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Actually, many real-time systems have both hard event and soft events mixed. The hardware resource (CPU time) and software resources (high priority interrupts) should be allocated to these processes handling hard-real-time events. 3. Networked embedded systems The networked embedded systems are connected to network with network interfaces to be accessed or to access other resources. The connected network can be Local Area Network (LAN), Wide Area Network(WAN), or Internet. The connection can be wired or wireless. You can simply group the networked embedded systems into wired and wireless sub-categories. But in many systems these two type systems are merged together. A home security system is an example of LAN networked embedded system where all sensors (e.g. motion detectors, press sensors, light sensors, or smoke sensors) are wired and running on the TCP/IP protocol. A home security system can be integrated into a web based security system with additional web camera running on HTTP protocol.

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Contd… Pervasive computing is the new trend of networked embedded systems application towards increasingly ubiquitous environment with all very tiny and even invisible embedded devices, either mobile or embedded in almost any type of thinkable objects where information and computational resources are organized as a ubiquitous service grid. Although you classify all embedded systems into these three major categories the division is not absolute. A sub-system of a networked embedded system can be real-timed or no-real-timed. A real-time system can be stand-alone or networked. Scale based categories. For example, small scaled embedded system supported by single 8-16 bit microprocessor or microcontroller with on-chip RAM and ROM is designed to perform simple task; medium scaled embedded system supported by a 16/32 bit microprocessor or microcontroller with external RAM and ROM to perform more complex operations; large scaled networked embedded system supported by 32- bit or 64-bit multiple chips which conduct distributed jobs.

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Characteristics of Embedded System Reliability is a very important character for embedded systems. Many systems work in mission critical and life-threatening environment where system reliability is crucial. Embedded systems are often embedded in machines that are expected to run continuously for years without faults, and tolerate the errors by themselves if an error occurs. Therefore the software is usually developed and tested more thoroughly than that for personal computers. Most embedded systems cannot be shut down for maintenance, or it is very difficult to repair such as aerospace systems. Some system will lose large amounts of money when shut down: Telephone switches, factory controls, bridge and elevator controls, funds transfer and market making, automated sales and service.

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The constraints of embedded system are also the challenges for the system design and development. Resource constraint: Embedded systems are constrained for their size, power capacity (may be battery operated), limited memory capacity(especially RAM size), CPU speed and function capacity Time constraints: Real-time system has response time deadline to meet. Environment constraint: extreme operation condition such as high or low temperature, high pressure, high humidity, under water, under collision, etc. Cost Constraint: Cost reduction of products has huge impact on the market competition because the large volume of products For high volume systems such as MP3 players or PDAs, minimizing cost is usually the primary design consideration. The selected hardware should be just “good enough” to cover the necessary functions. Time to market constraint: Design and development cycle is very limit in order to beat the competent in the market.

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Contd… Wide variety of chips for selection Many RTOS selections available, even the selection of using RTOS or not Language selections on high level or assembly Many I/O accessories components selections Programming on direct port operations Programming with multi-tasking and multi-threading execution Must use cross compiler and development tools Need special emulation tools to debug Testing on Multitasking and scheduling execution Testing on the real world environment because embedded software is embedded into devices.

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Overall Hardware Architecture Central Processing Unit (CPU) is the heart of microprocessor(μP) or microcontroller(μC). A microprocessor is a stand-alone CPU chip and memory and I/O ports can be custom designed and expanded. A microprocessor chip is a single integrated circuit intended to operate for general purpose and can be embedded into embedded electronics hardware. A microcontroller is an integrated chip which comes with built-in memory, I/O ports, timers, and other components. Most embedded systems are built on microcontrollers which run faster than custom built system with microprocessor because all components are integrated within a single chip. There is a wide variety of microprocessors and microcontroller available.

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The Intel 80C51(CMOS) microcontroller has a 8 bit microprocessor running at 1MIPS/12MHz with small on- chip RAM of 128 bytes for data and 4K on-chip ROM for code. It is integrated with 4 bi-directional I/O ports, two counter/timers, and Universal Asynchronous Receiver Transmitter(UART) for RS232 serial communication. It can support up to 64K external RAM and ROM. 8051 microcontroller to discuss due to its popularity, low cost, and especially the availability of many affordable IDE development kits such as the Keil μvision development kit and C8051F005DK development kits(the target board included) with support of C programming language.

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Memory Embedded system memory can come on-chip or off-chip. For same type memory, on-chip memory access is much fast than off-chip memory but the size of on-chip memory is much smaller than the size of off-chip memory. Usually, it takes at least two I/O ports as external address lines plus a few control lines such as R/W and ALE control lines to enable the extended memory. E.g., 16 bits of I/O port P0 and P2 are used to address 64k external memory in 8051. ( 2 16 = 2 6 x 2 10 = 64k byte memory space) RAM The 8051 memory is divided into Data Memory and Code Memory. Most of data are stored in Random Access Memory (RAM) and code are stored Read Only Memory ( ROM). This is due to the RAM constraint of embedded system and the memory organizations

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The RAM size of microprocessor and microcontroller is rather small (<256 bytes in most cases). In order to access any byte in a 256 bytes memory space, there need 8 address lines (internal address bus) because 2 8 = 256. One ASCII character takes one byte memory space. For example, character ‘A’ has its ASCII code 65 10, i.e., in binary 01000001 2 (8 bits = 1 byte); A integer number 1024 10 takes 11 bits(10000000000 2 ) which needs 2 bytes to store it. A simplest machine instruction itself takes one byte but some other memory access machine instruction take multiple bytes. The small amount of automatic variable data ( local variables of functions and interrupt service routine functions, stacks) are stored on the on-chip RAM for fast and frequent access and update while the large amount temporary data (array, table) can be stored in off-chip RAM. The RAM chip can be SRAM (static) or DRAM(dynamic) depending on the manufacture. SRAM is faster than DRAM but is more expensive RAM( Contd. )

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ROM The ROM, EPROM, and Flash memory are all read only type memory used to store code in embedded system. The embedded system code does not change after the code is loaded into memory. The ROM is programmed at the factory and can not be changed over time. The newer microcontroller comes with EPROM or Flash instead of ROM. Most of microcontroller development kits come with EPROM as well. EPROM and Flash memory are easier to rewrite than ROM. EPROM is a Erasable Programmable ROM in which the contents can be field programmed by a special burner and can be erased by a UV light bulb. The size of EPROM ranges up to 32kb in most embedded systems.

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All ROM type memory are non-volatile. This is one of the reasons the embedded code firmware is store in ROM type memory. When the power is on, the first instruction in ROM is loaded into the PC and then the CPU fetches the instruction from the location in the ROM pointed by PC and stores it in the IR to start the CPU fetch and execution cycle continuously. The PC is advanced to the address of next instruction depending on the length of the current instruction or the destination of the Jump instruction. ROM ( Contd. ) The 68HC11 comes with 256 bytes RAM, 512 bytes EPROM, and 8k bytes ROM. Here 1k is 1024 bytes (2 10 ) and 8k = 2 13 bytes. There need to be 13 address lines available to cover 8k bytes space. The 8051 comes with 128 bytes RAM and 4k ROM. The EPROM or Flash memory can be attached as extended memory.

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Bus All the CPU, memory, and I/O ports are connected by the buses. There are three type buses: data bus, address bus, and control bus. The bus is a pathway to connect all the microcontroller components A 8 bits or 16 bits data bus delivers data from one component to another component within the microcontroller. The address bus is used to specify the memory location for CPU to load instructions to be executed or to access data in the memory or ports. The control bus is used for CPU to control the operation in sequence with the timing tick paces and specify the read/write memory access operation. In order to run an instruction in the ROM, CPU needs to load the instruction from the ROM to IR based on the PC, extract the op-code and operands, load the operands from memory, perform the instruction operation in ALU, store the results in the destination, and starts new fetch-execution cycle, all of these are controlled by the control signals on the control bus issued by the CU. The external bus is used for microcontroller to connect to the external devices.

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I/O Ports The I/O ports are used to connect input and output devices. The common input devices for an embedded system include keypad, switch, button, knob, all kind sensors (light, temperature, pressure,..) The output devices include Light Emitting Diode(LED), Liquid Crystal display(LCD), print, alarm, actuator, etc. Both input and output such as communication interfaces include Network Interface Card (NIC), modem, and mobile phone Parallel Port A microcontroller is integrated with several parallel I/O ports which are used to interface the microcontroller to outside devices such as switches, LCD, keypad, and actuators. The parallel ports get or send 8 bits data at a time from/to connected outside devices. Many microcontrollers have 4-8 ports, e.g., both the 8051 and 68HC11 have 8-bit I/O ports.

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Some of the ports are input only(port E of 68HC11) to connect input devices such as keypad, sensors. Some of the ports are used for output only (Port D of 68HC11) to connect output devices such as LCD and any actuator. Some I/O ports are partial input/output like the port A in 68HC11. The bi-directional ports are the most common I/O ports in any microcontrollers. The 8051 I/O ports are all bi-directional. The multiplexers and demultiplexers facilitate multiple channel communications via a single common port.

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Serial Ports Some I/O ports such as the port D of 68HC11 and port3 of 8051 can be used to connect a external serial device. The serial communication exchanges data one bit at a time with single data line plus reference lines if needed. The typical application of serial port is to connect a PC with a serial cable to take advantages of PC for embedded software debug, testing, and deployments. A serial ports also allow data to flow between microcontrollers, or between microcontrollers and any serial device.

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Serial Communication There must be a protocol (rule) to governor the handshaking between two parties so that the receiver can recognize the beginning of the new coming byte and the ending of the byte. Assume the base logic is high “1” and once the transfer starts, the line is lowed to “0” for a period of time to signal the receiver that the start bit is coming soon, then the receiver activates its accepting circuit. Both sender and receiver follow the same clock pace so that the receiver can sample the right data. The logic “1” is resumed after the 8 bits delivered to mark the end of transmission of one byte data. The RS232 is a serial communication standard which specifies the handshaking rules between two sites. E.g., the communication of a 9-pin COM port connecting to a PC COM port with serial cable complies the RS232 protocol. The data is transferred at a specified baud rate (bps – number of bits per second). The baud rate at both sites must be set same so that the signals can be synchronized. It is called asynchronous communication because there is no clock control line between for transmitter and receiver synchronizes the pace.

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The asynchronous serial communication is support by a built UART circuit. It is possible to get and send data at same time in a full-duplex mode or at separate time in the half-duplex mode. In transmission the UART streams out 8 bits in sequence via TxD pin at the specified baud rate. After 1 byte data is out the UART set the TI flag to 1 to notify the completion of the transmission and makes the buffer ready for next byte data. The 68HC11 or 8051 provide TxD and RXD separate pins for microcontroller to send and get data to/from a connected serial device. The low left block diagram only depicts the UART in transition. The receiver works in the opposition direction via the RxD pin and the completion of receiving one byte is notified by the RI flag.

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Clock Oscillator, Timer and Watchdog The time is a very important factor to consider in embedded system design. All microcontrollers need an oscillator clock unit which generates the clock tick at a predetermined rate. The clock is used to control the clocking requirement of the CPU for executing instructions and the configurations of timers. For example, a classic 8051 clock cycle is (1/12)10 -6 second (1/12 µs) because the clock frequency is 12MHz. A simplest 8051 instruction takes one clock cycle(12 ticks = 1 µs) to complete. Of course some multi-cycle instructions take more clock cycles. A timer is a real time clock for real time programming. Every timer comes with a counter which can be configured by programs to count the incoming pulses. When the counter overflows(reset to zero) it will fire a time out interrupt that triggers predefined actions. Many time delays can be generated by timers. For example, a timer counter is configured to 24,000 and it will trigger the timeout signal in 24,000 x 1/12 µs = 2 ms.

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Clock Oscillator, Timer and Watchdog Contd… Each microprocessor provides multiple timers, the 8051 has two timers: Timer 0 and Timer 1. In addition to time delay generation, the timer is also widely used in the real time embedded system to schedule multiple tasks in multitasking programming. The watchdog timer is a special timing device that resets the system after a preset time delay in case of system anomaly. The watchdog starts up automatically after the system power up. You need to reboot your PC time by time due to various faults caused by hardware or software. An embedded system can not be rebooted manually because it has been embedded to its system. That is why many microcontroller comes with a on-chip watchdog timer which can be configured just as the counter in the regular timer. After a system gets stuck (power supply voltage out of range or regular timer does not issue timeout after reaching zero count) the watchdog eventually will restart the system to bring the system to a normal operational condition.

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Analog to Digital Converter (ADC) and DAC Many microcontroller kits integrate ADC and DAC units into the chip. Many embedded system application need to deal with non-digital external signals such as electronic voltage, music or voice, temperature, pressures, and many other signals in the analog forms. The digital computer does not understand these data unless they are converted to digital formats. The ADC is responsible to convert analog value to a binary number see the following diagram. DAC is responsible to output analog signals for automation controls such as DC motor or HVDC furnace control..

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Software Engineering Life cycle The specification and modeling, design, coding, testing, and integration The SDLC of a embedded system project can also be divided into two stages based on the system prototype paralleled with the hardware development. 1. Set up the model, map the model to code, and implement and test the code in a cross-platform development environment with the simulation development tools 2. Deploy the executable code produced in the host machine into the target machine (the embedded system itself) to run it on the target machine for the integration testing. Host and Target in Embedded Software Development

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The next chart shows the embedded C software compilation and link process in 8051 family microcontroller. Although more and more embedded software are developed by C but some time critical jobs are still better to be coded in assembly language for performance purpose. The C51 can mix the C code and assembly code by #PRAGMA directives( chapter 4) so that the C can embed inline assembly code or assembly routines when it is necessary. The SRC option of C51 compiler can produce the assembly source code from C source code. The C51 compile the C source code into object code and The A51 assembler assembles the assembly source code into objective code. The object code is not executable which has reference to library, external variable and external functions, and public symbol tables which help the linker to link all objective files and library file together to produce the absolute target code to run on the target. The bl51 is the locator/linker which links all relocatable objective file into a single executable machine code and locate it into the target memory space to be executed. Software Engineering Life cycle Contd…

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Choose RTOS and Programming Language Most embedded software is multi-tasking based software which supports multiple tasks running concurrently on the same CPU. One CPU can only run one instruction at a time but multiple tasks are running simultaneously in a macro view. E.g.,, A temperature control embedded system has a thermal sensor and control panel with keypad and LCD. The embedded system maintains the room temperature with a range of 70F ~ 80F. If the temperature goes up 80 + then a red LED is turned on, falls below 70 - F then a green LED is on; Both LEDs are off if the temperature is between up and lower bounds. The current temperature is always displayed on the LCD. Users can reset the upper and lower bounds by the keypad at any time. The software must run at least two tasks concurrently.

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The task1 mainly monitors the current temperature and takes action when the temperature goes beyond the limits. It samples the sensor every 100 ms. The task2 is ready to take user inputs from keypad to rest the temperature upper limit and lower limit. This simple embedded software deals with real time and multiple tasks. You can design this software in multi-tasking by Real time Operating System (RTOS) or using the interrupt mechanisms supported by all microcontrollers. There are many commercial or open source RTOS available for you to choose from based on the microcontroller systems. RTX51 is a RTOS which comes with the Keil development kit Choose RTOS and Programming Language Contd…

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Summary The foundational concepts of microprocessors and microcontrollers The hardware and software structure of embedded systems, categories and application of embedded systems, the constraints and design criteria and challenges in embedded system design and development. a road map of embedded software design and development including software modeling, software coding, cross-compilation, multi-tasking programming, and RTOS implementation. Two getting started labs with the C8051F005DK development kit (with target board) of Silicon Labs and stand-alone C51 Keil software development kit are provided to help you get start with embedded system design in aspects of both hardware and software.